Glutaminase and/or asparaginase are enzymes which hydrolyze glutamine and/or asparagine to convert them into glutamic acid and/or aspartic acid and ammonia, and it is well known that these enzymes are obtained from animals, plants and microorganisms. However, these enzymes are enzymes which act upon glutamine and/or asparagine in a specific fashion and cannot deamidate glutamine and/or asparagine in a peptide. Much less, they cannot deamidate γ and/or β-amido groups of glutamine and/or asparagine in a protein having larger molecular weight than that of a peptide. Still less, they cannot act upon glutamine and/or asparagine bonded in a protein state.
Also, transglutaminase is known as an enzyme which acts upon amido groups existing in a peptide state. This enzyme catalyzes the reaction of introducing an amine compound into protein by covalent bonding or the reaction of cross-linking the glutamine residue and lysine residue of protein via ε-(γ-glutamyl)lysine-peptide bonding, using the amido group of peptide-bonded glutamine as an acyl donor and the amino group of the primary amine as an acyl acceptor. It is known that, when amine or lysine does not exists in the reaction system or blocked, water acts as an acyl acceptor and the glutamine residue in peptide is deamidated to become glutamic acid residue, but since this enzyme is basically an acyl group transferase as described above, cross-linking reaction occurs when allowed to act on a usual protein and the reaction to deamidate protein does not occur, so that this enzyme is different from the enzyme of the invention.
In addition, Peptide glutaminase I and peptide glutaminase II produced by Bacillus circulans are known as an enzyme which performs deamidation by acting upon glutamine bonded in peptide. It is known that the former acts on the glutamine residue existing at the C terminal of peptide and the latter acts on the glutamine residue existing in the peptide. However, these enzymes do not act upon a high molecular weight protein and acts only upon a low molecular weight peptide [M. Kikuchi, H. Hayashida, E. Nakano and K. Sakaguchi, Biochemistry, vol. 10, 1222-1229 (1971)].
Also, plural studies have been made to attempt to allow these enzymes (Peptide glutaminase I and II) to act upon a high molecular weight protein rather than a low molecular weight peptide, and it has been revealed that these enzymes do not act on a high molecular weight protein but act only on a protein hydrolysate peptide. Illustratively, Gill et al. have reported that each of Peptide glutaminase I and II does not act on milk casein and whey protein both in native form and denatured form. They also have reported that, as a result of studies on activities on protein hydrolysate, only Peptide glutaminase II acted only on peptide having a molecular weight of 5,000 or less (B. P. Gill, A. J. O'Shaughnessey, P. Henderson and D. R. Headon, Ir. J. Food Sci. Technol., vol. 9, 33-41 (1985)). Similar studies were carried out by Hamada et al. using soy bean protein, and the result was consistent with the result by Gill et al. That is, it was reported that these enzymes showed deamidation percentage of 24.4 to 47.7% on soy bean peptide (Peptone), but did not substantially act on soy bean protein (0.4 to 0.8%) (J. S. Hamada, F. F. Shih, A. W. Frank and W. E. Marshall, J. Food Science, vol. 53, no. 2, 671-672 (1988)).
A series of these reports by Hamada et al. show data indicating that peptidoglutaminase derived from Bacillus circulans acts on protein though very slightly. On the other hand, Kikuchi et al. (M. Kikuchi, H. Hayashida, E. Nakano and K. Sakaguchi, Biochemistry, vol. 10, 1222-1229 (1971) and Gill et al. (B. P. Gill, A. J. O'Shaughnessey, P. Henderson and D. R. Headon, Ir. J. Food Sci. Technol., vol. 9, 33-41 (1985)) have used the same enzyme derived from the same strain (Bacillus circulans ATCC 21590) and reported that this enzyme acts on low molecular weight peptide but does not act on protein. The present inventor has purified the peptidoglutaminase derived from Bacillus circulans ATCC 21590 and confirmed that the slight apparent deamidation activity on protein reported by Hamada et al. is based on the action the enzyme upon peptide formed by the protease contaminated in the peptidoglutaminase preparation.
There is a report suggesting the existence of an enzyme originating from plant seed, which catalyzes deamidation of protein (I. A. Vaintraub, L. V. Kotova and R. Shara, FEBS Letters, vol. 302, 169-171 (1992)). Although this report observed ammonia release from protein using a partially purified enzyme sample, it is clear that this report does not prove the existence of the enzyme disclosed in the invention based on the following reasons. That is, since a partially purified enzyme sample was used, absence of protease activity was not confirmed, and no change in molecular weight of substrate protein after the reaction was not confirmed, there remains a possibility that not one enzyme but plural enzymes such as protease and peptidase acted on protein to release glutamine and/or asparagine as free amino acids and ammonia was released by glutaminase and/or asparaginase which deamidate these free amino acids or a possibility that glutamine-containing low molecular weight peptide produced in a similar way is deamidated by a peptide glutaminase-like enzyme. In addition, there is a possibility that deamidation occurred as a side-reaction by protease. In particular, it should be noted that this report clearly describes that a glutaminase activity which acted on free glutamine to release ammonia was present in the partially purified preparation used therein.
Accordingly, there is no report until now which confirmed the presence of an enzyme which catalyzes deamidation of high molecular weight protein, by purifying the enzyme as a single protein and isolating and expressing the gene encoding the same.
In general, when carboxyl groups are formed by deamidation of glutamine and asparagine residues in protein, negative charge of the protein increases and, as the results, its isoelectric point decreases and its hydration ability increases. It also causes reduction of mutual reaction between protein molecules, namely, reduction of association ability, due to increase in the electrostatic repulsion. Solubility and water dispersibility of protein sharply increase by these changes. Also, increase in the negative charge of protein results in the change of the higher-order structure of the protein caused by loosening of its folding, thus exposing the hydrophobic region buried in the protein molecule to the molecular surface. In consequence, a deamidated protein has amphipathic property and becomes an ideal surface active agent, so that emulsification ability, emulsification stability, foamability and foam-stability of the protein are sharply improved.
Thus, deamidation of a protein results in the improvement of its various functional characteristics, so that the use of the protein increases sharply (e.g., Molecular Approaches to Improving Food Quality and Safety, D. Chatnagar and T. E. Cleveland, eds., Van Nostrand Reinhold, New York, 1992, p. 37).
Because of this, a large number of methods for the deamidation of protein have been studied and proposed. An example of chemical deamidation of protein is a method in which protein is treated with a mild acid or a mild alkali under high temperature condition. In general, amido groups of glutamine and asparagine residues in protein are hydrolyzed by an acid or a base. However, this reaction is nonspecific and accompanies cutting of peptide bond under a strong acid or alkali condition. It also accompanies denaturation of protein to spoil functionality of the protein.
Because of this, various means have been devised with the aim of limiting these undesired reactions, and a mild acid treatment (e.g., J. W. Finley, J. Food Sci., 40, 1283, 1975; C. W. Wu, S. Nakai and W. D. Powie, J. Agric. Food Chem., 24, 504, 1976) and a mild alkali treatment (e.g., A. Dilollo, I. Alli, C. Biloarders and N. Barthakur, J. Agric. Food Chem., 41, 24, 1993) have been proposed. In addition, the use of sodium dodecyl sulfate as an acid (F. F. Shih and A. Kalmar, J. Agric. Food Chem., 35, 672, 1987) or cation exchange resin as a catalyst (F. F. Shih, J. Food Sci., 52, 1529, 1987) and a high temperature treatment under a low moisture condition (J. Zhang, T. C. Lee and C. T. Ho, J. Agric. Food Chem., 41, 1840, 1993) have also been attempted.
However, all of these methods have a difficulty in completely restricting cutting of peptide bond. The cutting of peptide bond is not desirable, because it inhibits functional improvement of protein expected by its deamidation (particularly reduction of foam stability) and also causes generation of bitterness. Also, the alkali treatment method is efficient in comparison with the acid treatment method, but it has disadvantages in that it causes racemization of amino acids and formation of lysinoalanine which has a possibility of exerting toxicity.
On the other hand, some enzymatic deamidation methods have also been attempted with the aim of resolving these problems of the chemical methods. Namely, a protease treatment method under a high pH (pH 10) condition (A. Kato, A. Tanaka, N. Matsudomi and K. Kobayashi, J. Agric. Food Chem., 35, 224, 1987), a transglutaminase method (M. Motoki, K. Seguro, A. Nio and K. Takinami, Agric. Biol. Chem., 50, 3025, 1986) and a peptide glutaminase method (J. S. Hamada and W. E. Marshall, J. Food Sci., 54, 598, 1989) have been proposed, but all of these three methods have disadvantages.
Firstly, the protease method cannot avoid cutting of peptide bond as its original reaction. As described in the foregoing, cutting of peptide bond is not desirable.
In the case of the transglutaminase method, it is necessary to chemically protect ε-amino group of lysine residue in advance, in order to prevent cross-linking reaction caused by the formation of isopeptide bond between glutamine and lysine, as the original reaction of the enzyme. When a deamidated protein is used in food, it is necessary to deamidate glutamine after protection of the ε-amino group with a reversible protecting group such as citraconyl group, to remove the protecting group thereafter and then to separate the deamidated protein from the released citraconic acid. It is evident that these steps sharply increase the production cost and are far from the realization.
In the case of the peptidoglutaminase method, on the other hand, since this enzyme is an enzyme which originally catalyzes only deamidation of low molecular weight peptide as described in the foregoing, its joint use with protease is inevitable, so that it causes problems of forming bitter peptide and reducing functionality (particularly foam stability) similar to the case of the protease method.
In consequence, though the reaction selectivity due to high substrate specificity of enzymes is originally one of the greatest advantages of the enzymatic method, which surpasses chemical and physical methods, it is the present situation that the enzymatic method cannot be put into practical use for the purpose of effecting deamidation of protein because of the absence of an enzyme which does not generate side reactions and is suited for the deamidation of high molecular weight protein.
Under such circumstances, an enzyme capable of deamidating protein without causing reduction of molecular weight of the protein has been called for in the field of food protein industry.
Accordingly, the inventor of the invention has established a screening method which can be applied broadly to the microbial world and, by the use of this method, succeeded in finding microorganisms which can produce an enzyme capable of deamidating protein without causing reduction of molecular weight of the protein broadly in the microbial world, and have accomplished the invention by carrying out culturing of the microorganisms and isolation and purification of the deamidating enzymes.